Stillorgan reservoir: maintaining stability during decommissioning and for new construction

Abstract

Operated by Dublin City Council on behalf of Irish Water, Stillorgan reservoir comprises three open cells (Gray, Upper and Lower reservoirs), created and separated by earth embankments, which store drinking water to supply the Greater Dublin area. To maintain a secure and sustainable supply of safe drinking water into the long-term, Irish Water plans to replace the open reservoirs with a single covered reservoir constructed within the Gray reservoir basin. This paper will discuss the analysis and methodology used to safely complete the drawdown of Gray reservoir while both Upper and Lower reservoirs remained operational. Stability modelling of the shared embankment between Gray and Upper reservoirs was undertaken to establish initial safe drawdown rates and to manage embankment stability until drawdown was completed. This included the interpretation and validation of piezometric data obtained during the drawdown period. Once the reservoir was emptied, vibration monitors and surface markers were installed to monitor the embankment during the preparation of the basin for construction of the service reservoir.